Pressure-sensitive switches are used in a variety of applications where it is desired to switch apparatus on or off at predetermined pressures. Switching may be desirable, for example, because the apparatus controlled by the switch may exceed its design limits, might be damaged, or give inaccurate and misleading readings when operated at extreme pressures. Certain government regulations also require that certain forms of commercial apparatus capable of both commercial and military uses, so-called dual use technology, be used with pressure sensitive switches to prevent them from being converted to military applications.
One important application of pressure-sensitive switches is in hydrophone streamer cable arrays used in underwater surveying. In such surveying, a survey ship tows a plurality of submerged cables extending substantially parallel to the ship's direction of travel. Each of the plurality (typically 4-10) of hydrophone cables is secured to one of a series of laterally spaced apart drums located on a ship's stern to keep them laterally separated so that they extend parallel to each other and to the ship's direction of travel. Additional lateral control is provided by paravanes associated with each cable to steer them as necessary. These hydrophone cables are of substantial length, up to 5000 meters. Each cable comprises a waterproof hollow elongate prismatic sheath, typically a hollow, flexible polymeric tube and at least one tensile member fixedly associated with the sheath; this tensile member providing structural integrity to the cable so that it will be not damaged by the substantial drag forces exerted upon the lengthy cable as it is towed through the water at speeds of several kilometers per hour. Commercial cables usually have three tensile members in the form of steel cables secured within the plastic tube at intervals of 120°. Hydrophones are secured within the plastic tube, inside the cables arranged at regular intervals along the axis of the tube, typically about 1 m. These hydrophones incorporate pressure detectors, normally piezoelectric detectors, capable of detecting sound pressure in the water caused by the explosions used in seismic surveying. The hollow interior of the tube is filled with oil so that vibrations in the water surrounding the cable are efficiently transmitted to the hydrophones. Electrical conductors extend the full length of the hydrophone cable to supply power to the detectors and to carry signal from the detectors back to recording and/or analysis equipment carried on the ship. Signal conditioning modules are usually included approximately every 300 m for amplification and signal conditioning such as filtering, if required.
Although commercial hydrophone cables are normally towed at depths of about 6 to about 25 meters during seismic surveying, the hydrophones they carry may operate down to 100 meters or more. As will be apparent to those knowledgeable in anti-submarine warfare, in the absence of any special precautions, a commercial hydrophone cable of the type already described would make an excellent submarine-hunting device, and international sales of such cables would have to be regulated under munitions control regulations. To permit international sales of commercial hydrophone cables and certain other dual-use technologies without cumbersome regulations, the United States and thirty-two other countries have concluded the Wassenaar Agreement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies. This Wassenaar Agreement, and the U.S. government regulations promulgated thereunder (see Commerce Control List, Part 774, Supplement No. 1, Category 6—Sensors and Lasers) provide that hydrophone cables may be freely sold provided they are equipped with pressure-sensitive switches such that the hydrophones will cease to operate at depths exceeding 35 meters. This somewhat arbitrary limit is the average value of the depth of the thermocline present in deep ocean waters. To be useful in anti-submarine warfare, hydrophones must be capable of operating below the thermocline. Further, the commerce control list states that the pressure switches should not be adjustable once installed in the tube.
Providing a suitable form of pressure-sensitive switch to meet this “cut-out” requirement of the Wassenaar Agreement has proved difficult. Such a switch must be inexpensive. In practice, each of the thousands of individual hydrophones in an array needs its own switch (commercial users prefer to buy the hydrophone and the switch as an integrated unit, since installing separate hydrophones and switches in a cable is complicated and too expensive), and since the price for the integrated unit cannot exceed about $30, the cost of the switch must be very low. The pressure at which the switch closes cannot deviate substantially from the desired 35 meter setting, since in practice the hydrophones within each cable are arranged in sections of (typically) 96 further arranged in groups of 8 (typically), and premature closing of any one switch deactivates the entire group of hydrophones, so that premature closing of a few switches among the thousands in an array may deactivate so many hydrophones that the value of the survey may be greatly reduced, or the survey may even have to be suspended while the affected groups of hydrophones are replaced. With the costs of survey ships running into thousands of dollars per hour, such downtime is highly undesirable.
In addition, it is desirable for any switch used with a hydrophone to not appreciably add to the overall volume of the combination since limited space is allocated for each hydrophone in an array assembly. Moreover, it is important to keep the hydrophone sensitive detection areas as far as possible from the noisy boundary layer at the external surface of the cable to enhance signal to noise ratios. Therefore, the switch should not alter any optimized hydrophone design that achieves this feature, and it is desirable for the switch to be acoustically isolated from the hydrophone and not alter its acoustic response characteristics.
Finally, although the cable is designed to surround the hydrophones with a non-corrosive oil, in practice sea water often leaks into a cable during extended commercial use, so the switch should be capable of resisting corrosion by salt water. Preferred embodiments of the switch of the present invention allow adjustment of the closing pressure of individual switches at the time of manufacture.
The present invention seeks to provide a pressure-sensitive switch which is very suitable for use in a hydrophone cable (although it may be used in numerous other applications) and which reduces or eliminates the aforementioned disadvantages of prior art pressure sensitive switches. The pressure-sensitive switches used in hydrophone array applications are generally of either the normally-closed or normally-open variety, usually connected electrically with an associated hydrophone in the array. An example of the normally-open variety overcoming many of the aforementioned disadvantages is described in related U.S. Pat. No. 6,318,497. In certain hydrophone array systems, it is convenient or desirable to incorporate normally closed pressure-sensitive switches. Certain systems are designed, for example, to limit a pressure-sensitive switches' disabling effect to an individual hydrophone without disabling large numbers of other hydrophones in a connected group. Such a circuit is described in related U.S. patent application Ser. No. 11/503,637 filed on Aug. 14, 2006.
The switch of the present invention is especially useful in conjunction with hydrophones of the types described in U.S. Pat. Nos. 5,646,470 and 5,675,556, which are substantially cylindrical with openings at each end through which the external pressure is transmitted to the interior of the hydrophone.
It is a primary object of the present invention to provide a normally closed pressure sensitive-switch.
It is another object of the present invention to provide a pressure sensitive switch for use in disrupting signals from hydrophones that descend beyond a predetermined water depth.
It is yet another object of the present invention to provide a pressure sensitive switch for use with arrays of hydrophones to selectively disrupt signals from selected individual hydrophones exceeding proscribed operating conditions.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter, when reading the following detailed description in connection with the drawings.